Mayor Bloomberg will be able to balance next year’s $70 billion budget because higher pension costs are being spread over 22 years — with the largest part of the bill coming due at the tail end, The Post has learned.

Sources said that arrangement means Bloomberg only has to spend an extra $575 million next year for pensions, instead of the $1 billion he had already set aside.

“It’s going to give them $425 million more for balancing the budget,” said one source. “That’s basically how they’re getting through this.”

The mayor had been projecting a deficit of $2 billion for the 2013 fiscal year, which starts on July 1.

Most worrisome was an anticipated $1 billion extra pension hit, which was predicated on the belief that city actuary Robert North Jr. was going to reduce the assumed rate of return for the five pension systems from 8 to 7 percent.

The lower the return rate, the more the city has to chip in to fund retirement checks.

But North found a way to spread out the liability over 22 years, providing Bloomberg with a windfall that’s being used to help close the $2 billion gap.

Sources said union officials who sit on the pension fund boards have yet to approve North’s strategy and may balk.

The mayor is presenting details of his preliminary spending plan for FY 2013 today.